Disk array data storage systems have multiple storage disk drive devices which are arranged and coordinated to form a single mass storage system. There are three primary design criteria for such storage systems: cost, performance, and availability. It is most desirable to produce memory devices that have a low cost per megabyte, a high input/output performance, and high data availability. "Availability" is the ability to access data stored in the storage system and the ability to insure continued operation in the event of some failure. Typically, data availability is provided through the use of redundancy wherein data, or relationships among data, are stored in multiple locations. In the event that a storage disk in the disk array partially or completely fails, the user data can be reconstructed via the redundant data stored on the remaining disks.
There are two common methods of storing redundant data. According to the first or "mirror" method, data is duplicated and stored in two separate areas of the Storage system. For example, in a disk array, the identical data is provided on two separate disks in the disk array. The mirror method has the advantages of high performance and high data availability due to the duplex storing technique. However, the mirror method is also relatively expensive as it effectively doubles the cost of storing data.
In the second or "parity" method, a portion of the storage area is used to store redundant data, but the size of the redundant storage area is less than the remaining storage space used to store the original data. For example, in a disk array having five disks, four disks might be used to store data with the fifth disk being dedicated to storing redundant data. The parity method is advantageous because it is less costly than the mirror method, but it also has lower performance and availability characteristics in comparison to the mirror method.
Apart from data redundancy, some disk array data storage systems enhance data availability by reserving an additional physical storage disk that can be substituted for a failed storage disk. This extra storage disk is referred to as a "spare." The spare disk is used to reconstruct user data and restore redundancy in the disk array after the disk failure, a process known as "rebuilding." In some cases, the extra storage disk is actually attached to and fully operable within the disk array, but remains idle until a storage disk fails. These live storage disks are referred to as "hot spares".
While the hot spare disk is useful in the event of disk failure, it provides no services to the disk array during normal operation when all disks are functioning properly. Reserving and earmarking an entire physical storage disk for hot spare purposes therefore increases costs to the system, without the benefit of fully utilizing the additional resources for data storage functions.
It would be advantageous to provide hot spare resources while still fully utilizing all storage disks of the disk array.